U.S. Pat. No. 4,722,605 issued Feb. 2, 1988 to Livnat et al. shows Moire distance measurement method and apparatus useful for noncontact measurement of small displacements of a specularly reflecting surface with a high degree of accuracy. The technique described by Livnat et al. involves projecting a collimated beam of light through a first grating onto the specularly reflecting surface. The light reflected by the surface is modulated by a second grating rotated at an angle .theta. with respect to the first grating to form a Moire pattern. A displacement of the surface causes the Moire pattern to shift. The shift in the Moire pattern is detected to measure the distance moved by the surface.
According to this method, a relatively large area of the surface being measured needs to be relatively flat and specularly reflective. This method is very useful for measuring the surfaces of liquids, and sheet materials such as plate glass. However, in an automated manufacturing environment it is often desirable to accurately measure the small displacement of parts that do not have relatively large specularly reflective surfaces.
Accordingly, it is the object of the present invention to provide a Moire distance measurement method and apparatus for measuring the displacement of a diffusely reflective surface.